The following relates to the nuclear power reactor arts, nuclear reaction control apparatus arts, control rod assembly arts, and related arts.
In thermal nuclear power plants, a nuclear reactor core comprises a fissile material having size and composition selected to support a desired nuclear fission chain reaction. The core is disposed in a pressure vessel immersed in primary coolant water. It is further known to control or stop the reaction by inserting “control rods” comprising a neutron-absorbing material into guide tubes passing through the reactor core. When inserted, the control rods absorb neutrons so as to slow or stop the chain reaction.
The control rods are operated by control rod drive mechanisms (CRDMs). In so-called “regulating” control rods, the insertion of the control rods is continuously adjustable so as to provide continuously adjustable reaction rate control. In so-called “shutdown” control rods, the insertion is either fully in or fully out. During normal operation the shutdown rods are fully retracted from the reactor core; during a SCRAM, the shutdown rods are rapidly fully inserted so as to rapidly stop the chain reaction. Control rods can also be designed to perform both regulating and shutdown rod functions. In some such dual function control rods, the control rod is configured to be detachable from the CRDM in the event of a SCRAM, such that the detached control rod falls into the reactor core under the influence of gravity. In some systems, such as naval systems, a hydraulic pressure or other positive force (other than gravity) is also provided to drive the detached control rod into the core.
To complete the control system, a control rod/CRDM coupling is provided. A known coupling includes a connecting rod having a lower end at which a spider is secured. The upper portion of the connecting rod operatively connects with the CRDM. In regulating rods, this connection includes a lead screw or other incremental adjustment element. Conventionally, the lead screw scrams with the connecting rod, spider, and control rods as a translating assembly (also known as the “control rod assembly”). In some known approaches, however, the lead screw may be retained in the CRDM such that only the connecting rod scrams. See, e.g. U.S. Pub. No. 2010-0316177 A1 published Dec. 16, 2010 which is incorporated herein by reference in its entirety, and U.S. Pub. No. 2011-0222640 A1 published Sep. 15, 2011 which is incorporated herein by reference in its entirety. To reduce cost and overall system complexity, a single CRDM is typically connected with a plurality of control rods via a spider. In this arrangement, all the control rods coupled with a single spider together as a translating control rod assembly (CRA). In practice a number of CRDM units are provided, each of which is coupled with a plurality of control rods via a spider, so as to provide some redundancy. The spider extends laterally away from the lower end of the connecting rod to provide a large “surface area” for attachment of multiple control rods.
The translating CRA (including the control rods, spider, connecting rod, and optionally also the lead screw) represents a substantial mass that falls under the force of gravity during a scram. It is advantageous for the translating CRA to have substantial mass in order to provide the driving force for the scram. In some designs, the translating CRA has a mass of a hundred pounds to a few hundred pounds, and may reach a terminal velocity of 10 feet per second or higher. Thus, consideration is given to the termination of the scram, that is, to the slowing and stopping of the downward falling of the translating assembly at the end of the scram event.
Prior to termination of the scram, the descending control rod tips engage dashpot tubes of narrowed inner diameter that produce a slowing force via a piston effect. Alternatively, a dashpot can be located in the CRDM which engages with the descending connecting rod or lead screw. Although such dashpots can provide some cushioning, the ultimate “stop” for the scram is impact of the descending spider onto the top of the fuel assembly (or onto a structural plate located above the fuel assembly). To cushion this final impact, it is known to employ one or more helical springs disposed in the connecting rod and/or spider. However, it is difficult to insert long springs into the translating CRA, and shorter springs do not provide large energy absorption. As a consequence, a substantial portion of the kinetic energy of the translating CRA is ultimately absorbed by the impact of the spider onto the fuel assembly, which can lead to damage to these critical components.